If you are breeding harem style and the dead are thin, you may have overbred females that can't cope with the strain.
If it is random rats of all ages(except for the adult males), I would suspect the food. Sometimes when I've had to feed dog food or something other than the regular rat block, I have rats just fall over dead and they always seem thin too.
Just my own observations with my colony.
If you have a lot of sneezing, then first guess will be that myco, which can kill a lot of your rats. I treated with livestock antibiotics from Tractor Supply the one time I had it run through my colony. I still lost quite a few but haven't had an issue with it since.
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